1. Field of the Invention
The present invention relates to a fuel pressure control apparatus for controlling the fuel pressure in an accumulator to a target pressure suitable for a vehicle situation in an internal combustion engine in which fuel that is accumulated in the accumulator is injected into a combustion chamber from a fuel injection valve.
2. Description of the Related Art
In what is called a direct injection internal combustion engine in which high-pressure fuel is directly injected into its combustion chamber, the injection characteristics such as the jet shape, momentum, particle size, and injection rate of fuel jetted from a fuel injection valve vary with the fuel pressure. To improve the combustion characteristics, fuel efficiency and, exhaust gas performance of the internal combustion engine, it is required to variably control the fuel pressure to a pressure that is most suitable for an operation statuses of the internal combustion engine. To satisfy this requirement, various fuel pressure control apparatus have been proposed in which the fuel pressure in an accumulator is controlled to a target pressure by controlling the amount of fuel that is supplied from a high-pressure fuel pump to the accumulator with an electromagnetic control valve for each cycle of the high-pressure fuel pump.
For example, JP-A-11-324757 (pages 4-6, FIGS. 1-7) discloses the following technique. In a direct injection type fuel injection device, the fuel supply amount of each time is controlled by feedback-controlling a pressure regulator that is provided on a fuel supply path on the basis of a target fuel pressure that is set in accordance with operation conditions of an internal combustion engine and a detection value of the fuel pressure in an accumulator and feedforward-controlling the pressure regulator so that an amount of fuel corresponding to that supplied from a fuel injection valve to an internal combustion engine is supplied from a fuel pump to the accumulator. As a result, the fuel pressure in the accumulator is caused to follow the target fuel pressure with high responsivity.
JP-A-61-96162 (pages 2-4, FIGS. 1-7) discloses the following technique which is directed to a fuel injection device for a diesel engine. A relief valve whose degree of opening varies linearly with the current is provided in a high-pressure fuel passage from a high-pressure fuel pump to a fuel injection valve. The fuel pressure in the high-pressure fuel passage is controlled by supplying a current to the coil of the relief valve in accordance with a detection value of the fuel pressure in the high-pressure fuel passage and engine operation statuses. As a result, during idling or a light-load operation, the fuel injection rate is lowered to reduce the noise and vibration. During a heavy-load operation, the injection rate is increased by setting the relief pressure higher than a pressure generated by the high-pressure fuel pump.
In the conventional fuel pressure control apparatus of JP-A-11-324757, although a control for increasing the fuel pressure in the accumulator can be performed easily, a control for lowering the fuel pressure cannot be performed actively because of the configuration of the apparatus. That is, although the fuel pressure in the accumulator can be increased by supplying fuel the amount of which exceeds an injection amount of the fuel injection valve, even if the amount of fuel discharged from the fuel pump is lowered or the fuel discharge is stopped the fuel pressure in the accumulator does not decrease until fuel that is injected from the fuel injection valve to the combustion chamber of the internal combustion engine is consumed. A state that the pressure in the accumulator does not coincide with a target fuel pressure continues for a time that is taken to consume the fuel.
In the control of JP-A-11-324757, a target fuel pressure is determined according to a two-dimensional map having the rotation speed and the load of the internal combustion engine as parameters and the target fuel pressure is set higher as the rotation speed or the load of the internal combustion engine increases. When the internal combustion engine is started in a state that such a target fuel pressure map is set, the rotation speed and the load vary in the following manner. When the ignition switch is turned on, the internal combustion engine is driven by the starter motor and the rotation speed is increased to a start rotation speed. The load of the internal combustion engine increases rapidly during a period from the turning-on of the ignition switch to complete combustion (i.e., a start period). Start fuel suitable for the load is supplied from the fuel injection valve. And fuel suitable for the load is supplied from the fuel pump to the accumulator, whereby the fuel pressure in the accumulator increases to a target fuel pressure.
Once complete combustion occurs in the internal combustion engine, the rotation speed converges to an idling rotation speed after a temporal increase and the load decreases accordingly. As a result, the target fuel pressure decreases rapidly according to the target fuel pressure map. However, since as described above the decrease in accumulator pressure depends on the fuel consumption by the discharge from the fuel injection valve, the accumulator pressure becomes higher than the target fuel pressure. That is, a state that the actual fuel pressure does not coincide with the target fuel pressure though the operation state of the internal combustion engine has converged to the idling state continues for a prescribed time. Because an operation state at a fuel pressure that does not provide optimum combustion performance continues for the prescribed time, deteriorated combustion performance may cause misfires etc.
In the control of JP-A-61-96162, a target fuel pressure is determined according to a two-dimensional map having the rotation speed and the throttle position of the internal combustion engine as parameters. Therefore, for example, the target fuel pressure is set higher as the rotation speed or the degree of opening of the throttle valve of the internal combustion engine increases. If with such settings the internal combustion engine is gunned in an idling state, the rotation speed of the internal combustion engine is increased in accordance with the accelerator manipulation and the target fuel pressure is switched according to the target fuel pressure map. As a result, a large amount of fuel is supplied from the high-pressure pump to the accumulator and the fuel pressure in the accumulator increases toward the new target fuel pressure.
However, when a gunning manipulation is performed, the throttle valve is opened and then closed immediately. The rotation speed of the internal combustion engine starts to decrease toward an original idling rotation speed. As a result, the target fuel pressure decreases to a value corresponding to the original idling rotation speed and the actual fuel pressure in the accumulator becomes higher than the target fuel pressure. Therefore, a state that the actual fuel pressure does not coincide with the target fuel pressure though the operation state of the internal combustion engine has already converged to the idling state continues for a prescribed time. Because an operation state at a fuel pressure that does not provide optimum combustion performance continues for the prescribed time, deteriorated combustion performance may cause misfires etc.